This invention relates to motion-compensated de-interlacing.
Traditional television systems use 2:1 vertical sub-sampling interlaced scanning. Interlacing causes artifacts, such as flicker, shimmering, and diagonal edge effects. The artifacts can be reduced by transforming the interlaced fields to progressive frames. FIG. 1 shows two interlaced frames in which the fields have temporal relationships suggested by the names top field_first and bottom_field_first.
In FIG. 2, the input of the de-interlacer is an alternating sequence of top and bottom fields of successive frames and the output of the de-interlacer for each pair of input fields are two progressive frames that represent the same image as the input fields but at higher vertical resolution.
Mathematically, the output frame of the de-interlacer is defined as ##EQU1##
where f.sub.o denotes an output progressive frame for the respective input interlaced field (top or bottom), fdenotes the current input field (top or bottom) to be de-interlaced, f.sub.i denotes an interpolated field of line samples to be merged with the lines of the input field, n.sub.x,n.sub.y designates the spatial position, i.e., the sample position along a line and the line in the field, and n.sub.t designates the field number. The top equation applies to input top fields and the bottom equation to input bottom fields.
De-interlacing doubles the vertical-temporal sampling density and aims at removing the vertical-temporal alias caused by the interlacing process. Unhappily, the de-interlacing process does not follow the conventional linear sampling rate theory because TV cameras do not perform a vertical-temporal low-pass filtering operation before sub-sampling the video signal into the odd-even vertical-temporal sampling pattern.
One simple way to de-interlace an input field is to vertically interpolate the missing samples in the incoming field. This approach yields blurred edges, does not eliminate temporal aliasing, and can produce line jittering in the resulting frames. Yet vertical interpolation may be used as a fallback mechanism when more sophisticated de-interlacing fails to produce a frame or a part of a frame with acceptable quality.
De-interlacing using motion compensation tries to interpolate in the direction of motion trajectory. Motion compensation thus potentially permits converting a moving scene into a stationary one. Motion-compensated de-interlacing produces an optimum de-interlacing strategy for motion that has a constant and linear velocity. There are several known ways to include motion compensation in de-interlacers.